The present invention generally relates to controlling transducer movement and, more particularly, to controlling transducer movement responsive to a position error signal within a servo control loop.
A typical data storage disk drive includes a plurality of magnetic recording disks which are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent to surfaces of the disks to transfer data between the disks and a host device. The heads can be radially positioned over the disks by a rotary actuator and a closed loop servo system.
The servo system can operate in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a target track on the corresponding disk surface. Upon reaching the target track, the servo system enters the track following mode wherein the head is maintained over the center of the target track while data is written/read. During track following, prerecorded servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the position error of the head away from the track center. The PES is then converted into an actuator control signal, which is fed back to an actuator to position the head.
In general, there are two forms of head positioning errors: repeatable and non-repeatable. Repeatable errors, which are generally caused by mechanical irregularities in the structure of the disc drive or errors introduced when writing the servo tracks, can be predicted and therefore theoretically can be cancelled out as they occur. In general, these repeatable runout errors (RRO) are removed by introducing a compensation signal into the servo loop that cancels the repeatable positioning error. Techniques for generating such compensation signals are generally referred to as feedforward cancellation.
Nonrepeatable runout (NRRO) disturbances can arise as a result of a variety of factors and the characteristics of the disturbance often vary depending on the source of the disturbance. Disc flutter disturbances have high energy concentrated at specific frequencies. Suspension windage-induced disturbances occur at higher frequencies and have wider bandwidths. Also, unlike RRO errors, NRRO errors do not occur with predictable magnitudes or frequencies. Moreover, the magnitude and frequency of NRRO error generally changes over time.